If you’re an average, 180-pound person, all the capillaries in your body – the smallest blood vessels, where oxygen and other nutrients are exchanged with cells – can together hold about 3 gallons of blood.

But blood, like water, is heavy. So you evolved into an evolutionary compromise. Your body only contains about 1.5 gallons of blood at a time; much lighter to carry, but only half of what you need to provide for your whole body at once. Fortunately, your body also evolved a smart system of hemodynamics, a combination of forces that sends that blood to capillaries as it’s needed.

At the front end, your heart pushes oxygen- and nutrient-rich blood through your arteries.

Then the movement of your muscles pulls that blood from your arteries into your capillaries, to feed individual cells.

In other words, while your heart is circulating blood all the time, the oxygen and nutrients only make it to cells when the muscles around them are moving.

That’s one of the major problems with excessive sitting: without movement, your cells are starving.

But that’s just one problem. After 30 minutes of sitting, your metabolism slows down by 90%. A few hours in, you’ve got increased blood triglyceride and insulin levels, and reduced (good) HDL cholesterol and lipoprotein lipase (an enzyme that breaks down fat in your body).

So perhaps it shouldn’t be surprising that people who sit more are sicker and fatter than people who don’t.

What’s more, that’s independent of exercise. Even between people who work out for the same number of hours weekly, a greater number of hours spent sitting each day correlates with an increase in both body mass and all-causes mortality. Studies have tied sitting to huge increases in everything from type 2 diabetes to cardiovascular disease and cancer.

For example, excess daily sitting increases your risk of lung cancer even more than the second-hand-smoke effects of living with a smoker.

All of which is bad news, because we apparently really love to sit. The average desk worker spends 7-8 hours a day sitting at the office, then comes home to sit down for another 5 hours of daily TV.

Fortunately, the solution is simple: get up frequently and move around.

Research has shown that even short breaks (a couple of minutes) at low intensity (walking to the bathroom, or simply standing up) make a huge difference. One study showed that, the greater the number of breaks taken, the lower the waist circumference and BMI, and the better the blood lipids and glucose tolerance.

Of course, once you get into the flow of work, it’s easy to forget just how much you’re sitting. That’s why you need a gentle nudge.

Breaktime for Mac or Rest for Windows will take over your screen at whatever interval you select, reminding you to stand up, shake it out, go the bathroom, grab a water or coffee, or similarly get that mini-dose of movement it takes to get your body back on track.

Getting up and moving every 30 minutes is a pretty small habit. But it pays big dividends in your short- and long-term health.